Phase-change materials exhibit at least two different states. The states of phase-change material may be referenced to as amorphous and crystalline states. The states may be distinguished because the amorphous state generally exhibits higher resistivity than does the crystalline state. Generally, the amorphous state involves a more disordered atomic structure, while the crystalline state is an ordered lattice.
Phase change in the phase-change materials may be induced reversibly. In this way, the phase-change material may change from the amorphous state to the crystalline state, and from the crystalline state to the amorphous state, in response to temperature changes. The temperature changes to the phase-change material may be achieved in a variety of ways. For example, a laser can be directed to the phase-change material, current may be driven through the phase-change material, or current can be fed through a resistive heater adjacent the phase-change material. With any of these methods, controllable heating of the phase-change material causes controllable phase change within the phase-change material.
The heating current needed to reach a fixed temperature such as the crystallization or melting temperature of the phase-change material varies considerably with the resistivity of the phase-change material. Typically, the electronic circuitry for providing the heating current is designed to work with a specified range of resistivities. Consequently, determination and control of resistivity is essential for successful device performance.
Typically, resistivity is determined by using a four point probe. The four point probe physically contacts the material to be tested. For production wafers, physical contact may contaminate the product wafer and/or damage the surface of the product wafer. Therefore, wasteful monitor wafers are typically used in place of product wafers if four point probe measurements are to be performed to measure resistivity. In addition, the probes typically have a spacing of approximately one millimeter. Therefore, several millimeter sized areas are dedicated for measurements on the wafer surface when using a four point probe. Four point probe measurements are also often performed at the edge of the wafer.